The most common method of providing line isolation and utility grid primary power backup is the use of uninterrupted power systems (UPS) as a secondary power source for providing sufficient power duration until motor generator units are running and the power stabilized. Such a system can provide utility-grid primary power backup, as indicated in FIG. 1. Common types of motor generator units are internal combustion engines (ICs), gas turbines (GTs), and microturbines (MTs). Each of these types of motor generator units have finite start up times and other issues as indicated in Table I. The negative issues of motor generators as indicated in Table I are the high unit cost, high fuel consumption cost, low electrical efficiencies, high operating and maintenance costs (O&M), and environmental issues associated with their emissions.
Other hidden costs involved in the installation of motor generators for primary power utility-grid power backup include expenses and length of time to obtain permits for these power sources. Concerns about environmental issues make obtaining such permits more difficult and in some areas around the world, impossible.
TABLE ICharacteristics of Motor Generator TechnologiesInternalCombustionGas TurbinesMicroturbinesTechnology(IC) Engine(GT)(MT)Cost:Unit Cost ($/kW)300-900300-1000700-1100O&M Cost ($/kWh)0.007-0.0150.004-0.0100.005-0.016Other Characteristics:Fuel TypeNatural gasNatural gasNatural gasEquipment life20 years20 years10 yearsStart-up times (cold start)10 secs10 mins2-5 minsElectrical efficiency30-37%22-37%23-28%Available (%)91.2-95.890-93.395Emissions:NOx (lb/MWh)4.71.150.44SOx (lb/MWh)0.4540.0080.008PM-10 (lb/MWh)0.780.080.09
UPS systems conventionally use stored energy as a secondary power source to protect critical loads and provide sufficient time to switch motor generators on-line to assure limited loss in power to the user when utility grid power is lost. The reliability of such stored energy is fundamental to the reliability of the system. Lead-acid batteries are the most commonly used type of stored energy in UPS systems. For a variety of reasons, including their ability to sustain deep charge/discharge cycles, as compared to common lead-acid batteries, Valve-Regulated Lead-Acid (VRLA) batteries are predominantly used. But despite battery manufacturers' best efforts to improve their products, experience has shown that the useful life of a VRLA battery array in conventional double-conversion UPS systems is two to three years. Beyond two years cell failure rates quickly reach unacceptable levels.
Electrochemical batteries prematurely reach end of life for two reasons: manufacturing defects and battery management issues. Manufacturing defects include “cold” welds between adjacent cells; inter-cell shorts; reversed plates; incomplete casting of the “straps,” which results in “dropped” plates; defects in paste mixing, which leads to poor paste adhesion; and contamination of the paste or electrode. Detecting defects in batteries after assembly is very difficult and expensive. Quality cannot practically be inspected-in after construction.
To overcome such problems of lead-acid batteries, a battery management strategy is tailored to the type of battery and its application. VRLA batteries in conventional double-conversion UPS systems, as indicated in FIG. 2, have limitations in the degrees of freedom that can be employed for battery management. Batteries in such systems are always “float” charged. That is, they are continuously supplied with a low charging voltage, in the range of 13.4 to 13.6 volts per jar, as a result of their connection to the DC bus of the rectifier/inverter combination, which is always active. The circuit topology also provides a level of DC ripple current which continuously flows in the batteries from the rectification/inversion process. As a result, a significant amount of heat is continuously generated in the batteries. VRLA batteries employ Absorbed Glass Mat (AGM) or gel electrolyte technology, in which the electrolyte is not free to move, making heat removal more difficult.
In contrast, some systems utilize maintenance-free flooded lead-acid batteries made with lead-calcium grids, in an off-line circuit topology that does not subject the battery string to continuous float charging or ripple currents. Batteries are not subjected to a continuous float charge. Instead they are periodically recharged by means of two different regimens: normal charge and equalize charge.
Temperature control is of concern in a battery management strategy. High temperature accelerates corrosion that destroys a battery's capacity to generate current.
Limiting the depth of discharge is utilized in controlling electrolyte stratification in flooded lead-acid batteries. Stratification is the increase of electrolyte specific gravity at the bottom of the battery.
Stratification can also be reduced by agitating the electrolyte. In vehicle applications, agitation occurs naturally with the movement of the vehicle. In stationary applications, agitation can be accomplished by periodically “gassing” the battery, by raising the charging voltage to the level where electrolysis of water occurs and hydrogen and oxygen gases are created within the electrolyte. The gas bubbles, moving through the electrolyte, provide a stifling action. The technique for providing this function is called equalize-charging which is used to replenish the energy losses in the battery due to self-discharge and operate the control function of the battery management system. However, if the equalize-charge process is performed too frequently, the oxygen generated from electrolysis can accelerate positive grid plate oxidation. Such oxidation can cause the plates to “grow” and short to the negative straps causing battery failure. Another reason to control the number of equalize charges is electrolysis of the water during each charging process diminishes the amount of water in the electrolyte, which can also lead to battery failure.
Other problems associated with lead-acid battery technology include low energy density, typically 70.2 W·h/L; low specific energy, typically 46.0 W·h/kg; high discharge rate; high life reduction with deep cycle use; high energy storage reduction with temperature; and it contains hazardous material.
The use of the same reference symbols in different drawings indicates similar or identical items.